Over last decades, several studies have been focused on short-term high light stress in lichens under laboratory conditions. Such studies reported a strong photoinhibition of photosynthesis accompanied by a partial photodestruction of PSII, involvement of photoprotective mechanisms, and resynthetic processes into gradual recovery. In our paper, we applied medium [800 µmol(photon) m -2 s -1 ] light stress to induce negative changes in PSII funcioning as well as pigment and glutathione (GSH) content in two Antarctic fruticose lichen species. Chlorophyll (Chl) fluorescence parameters, such as potential and effective quantum yield of photosynthetic processes and fast transients (OJIP) recorded during high light exposition and recovery, revealed that Usnea antarctica was less susceptible to photoinhibition than U. aurantiaco-atra. This might be supported by a more pronounced high light-induced reduction in Chl a and b contents in U. aurantiaco-atra compared with U. antarctica. In both experimental species, total GSH showed an initial increase during the first 30-40 min of high light treatment followed by a decrease (60 min) and an increase during dark recovery. Full GSH recovery, however, was not finished in U. aurantiaco-atra even after 5 h indicating lower capacity of photoprotective mechanisms in the species. OJIP curves showed high light-induced decrease in both species, however, the recovery of the OJIPs shape to pre-photoinhibitory values was faster and more apparent in U. antarctica than in U. aurantiaco-atra. The results are discussed in terms of sensitivity of the two species to photoinhibition and their photosynthetic performance in natural environment.

Optimum growth temperature of Trebouxia sp. (re-classified as Asterochloris sp. recently), a symbiotic lichenized alga was evaluated using a batch culture cultivated in a bioreactor. The algae were isolated from lichen thalli of Usnea antarctica collected at the James Ross Island, Antarctica in February 2012. The algae were isolated under laboratory conditions and then cultivated on agar medium at 5°C. When sufficiently developed, the algae were suspended in a BBM liquid medium and cultivated in a photobioreactor for 33 days at either 15, or 10°C. During cultivation, optical density (OD) characterizing culture growth, and effective quantum yield of photosystem II ( PSII ) characterizing photosynthetic performance were measured simultaneously. Thanks to higher  PSII values, faster growth was achieved at 10 o C than 15 o C indicating that Trebouxia sp. might be ranked among psychrotolerant species. Such conclusion is supported also by a higher specific growth rate found during exponential phase of culture growth. The results are discussed and compared to available data on temperaturedependent growth of polar microalgae.Key words: Usnea antarctica, chlorophyll fluorescence, lichen, effective quantum yield, James Ross Island, psychrotolerance, Asterochloris Abbreviations:  PSII -effective quantum yield of photosystem II,  -specific growth rate, OD -optical density, PBRr -photobioreactors